The present invention relates to a mechanism for reversing the movement of the inked ribbon of a typewriter or other printing office machine, wherein the ribbon is wound on a pair of spools controlled by a pair of corresponding rotors, and wherein a reversing member is adapted to be shifted under the control of ribbon sensing levers between first and second positions in which drive is imparted to the two rotors respectively.
A reversing mechanism is known wherein the reversing member is constituted by a rocking lever provided with an edge having two inclined surfaces converging towards an apex. A pin carried by a positioning lever co-operates with one of these inclined surfaces under the action of a spring which holds the rocking lever in this way in a first position for feeding the ribbon in a first direction. When a sensing lever bearing against the turns of the spool then acting as a take-up spool senses that this spool is nearly full, it urges the rocking lever to turn towards a second position against the action of the spring in question. The pin has to ride over the apex and, when it does so, the spring urges the pin against the other inclined surface and completes the shifting of the rocking lever into the second position to feed the ribbon in the opposite direction to the first direction. In similar manner, a second sensing lever and the spring shift the rocking lever from the second to the first position for another reversal of the movement when the second sensing lever senses that the other spool is nearly full. The rocking lever is provided with two projections which enable a pawl to rotate the corresponding rotors, in the form of toothed wheels fast with the spools in the respective directions. This mechanism requires the action of the spring of the positioning lever to prevail over that of the pawl. Moreover, in order that that positioning spring may shift the rocking lever, it is necessary for each sensing lever to load the positioning spring during the reversing stage so as to bring the pin over the apex. In this stage, the pressure exerted by the sensing element on the ribbon is considerable, so that the end portions of the ribbon are subjected to unacceptable wear. Moreover, the mechanism is complicated and therefore costly.
In U.S. Pat. No. 3,889,795, there is described an improved mechanism in which the rocking lever is merely unlatched by the sensing levers which do not, therefore, have to generate a large force and bear with little pressure on the ribbon. Moreover, each sensing lever unlatches the rocking lever when it senses that the spool acting as the feed spool is nearly empty and the empty diameter of a spool is more determinate than the full diameter. However, it is necessary, as each reversal takes place, to reload the rocking lever in the sense required for the next reversal and the mechanism which performs this action, driven from the rotor driving the spool acting as take-up spool, adds some additional costs to the structure.